Optical amplifiers such as fiber amplifiers using an erbium-doped fiber (known as erbium-doped fiber amplifiers (EDFA)) are used in optical transmission systems in order to amplify an optical payload signal that has been so severely attenuated after passing through a correspondingly long transmission path that an amplification of the signal is necessary. The direct amplification of the optical signal can be performed in an optical amplifier without the necessity of first performing an optoelectrical conversion and then an electrooptical conversion of the signal after signal amplification, as is the case for electrical amplification of the optical signal. The pumping power required in an optical amplifier is usually generated by means of a pump source having a laser diode, wherein the center wavelength of this pumping laser diode lies, for instance, in the range of 980 nm or in the range of 1480 nm. The pumping power is coupled by means of a coupler into the transmission path between the input port and the output port of the optical amplifier in the direction towards the optical amplification medium, for instance, an erbium-doped pumping fiber. The pumping power is absorbed by the erbium atoms; the optical signal to be amplified, which likewise passes through the pumping fiber, excites the erbium atoms, which have been excited to an elevated energy level, to produce a stimulated emission. The light generated in this manner thus has substantially the same frequency and the same phase as the optical payload signal to be amplified, which is thereby amplified in a purely optical manner. The optical gain G, which is defined as the ratio of the power of the amplified optical signal Pout available at the output port divided by the optical power Pin supplied to the input port, displays among other things a dependency on the degree of erbium doping and on the optical pumping power. The higher the pumping power Ppump, the greater is the optical gain G.
It must be taken into account, however, that in order to amplify an optical input signal Pin of high power, a larger pumping power is also necessary in order to obtain the same gain G. The pumping power Ppump generated by the pumping laser diode is directly proportional, to a good approximation, to the electrical pumping current Ipump flowing through the pumping laser diode, i.e.:Ppump=α·Ipump where α designates the proportionality constant. It must naturally be taken into account that this relationship does not apply to very small pumping currents. For the range of optical pumping power Ppump that is of interest, however, this relation represents a good approximation.
Since a part of the electrical power supplied to the pumping laser diode is converted into thermal loss power, the pumping current is limited to a maximum value Ipump,max. If this threshold for the pumping current is exceeded, the pumping laser diode is destroyed. Thus, the maximum possible optical pumping power is also limited to a maximum value Ppump,max.
In an optical amplifier, the pumping power Ppump is typically regulated as a function of the power of the optical signal Pin to be amplified and of the power of the optical output signal such that a predetermined value for the gain G is obtained. If the input optical signal power Pin supplied to the input port of the amplifier is increased, for example because additional optical channels are added to an optical wavelength division multiplex transmission system, then the required optical pumping power Ppump and thus the pumping current Ipump must also be raised in order to keep the optical gain G at the desired value.
It must be taken into account in this regard that the pumping laser in particular is subject to aging effects, which are summarized under the term degradation. In other words, the pumping current Ipump must be increased with increasing aging of a pumping laser diode in order to keep the optical pumping power Ppump at a constant value. This circumstance is taken into account in the aforementioned proportionality relationship by introducing a time variance of the proportionality constant α(t). This situation is evident from the schematic representation in FIG. 2. This representation shows the aging effect of a pumping laser diode, which has the steepest correlation between the parameters Ipump and Ppump at a time t0, and the corresponding correlation between these parameters at a later time t2 has the lowest slope. In other words, α(t) decreases over time.
If the degradation of the pumping laser diode increases over time to such an extent that the pumping current has reached the maximum value Ipump,max and can no longer be increased, in order to avoid a destruction of the pumping laser diode, then the gain G of the amplifier is reduced, whereby the performance of the optical transmission system in question is worsened. With typical well-known optical amplifiers for optical transmission paths, an alarm signal is therefore generated when the maximum value Ipump,max for the pumping current, or a slightly lower value that guarantees a certain margin of safety, has been reached. For optical transmission systems in which an optical amplifier is initially operated well below the maximum permissible pumping current Ipump,max, no alarm is issued, even if the pumping laser diode has already aged considerably, but a pumping current Ipump<Ipump,max still suffices to achieve the predetermined gain G.
However, if the optical transmission system and the optical amplifier were designed from the start for the amplification of larger input powers at the input of the optical amplifier, then it can occur that after a certain aging time, the maximum possible pumping power Ppump,max of the pumping laser diode is no longer sufficient to maintain the predetermined value G of the gain within the original specification if there is an increase of the optical input power Pin supplied to the amplifier. This can occur, for instance, in a wavelength division multiplex transmission system if additional channels are added after a certain aging time. Since the maximum possible pumping power Ppump,max no longer suffices, even the originally present channels are impaired. This is not acceptable, however.